Techniques for representing the spatial distribution of radioactive elements are today indispensable, and are commonly used in the field of biological and medical research.
Amongst the techniques currently used, the most common consists in applying a sensitive film onto the surfaces emitting electrons (termed primary electrons), or .beta. radiation, from these radioelements.
Various other techniques have the purpose of replacing the use of a sensitive film, and most of these techniques can claim to be a great improvement with regard to sensitivity, linearity and working dynamic range over those using a sensitive film.
Amongst these other techniques, the one termed the "erasable phosphor technique" is currently the preferred technique in numerous research organizations and laboratories.
It should be recalled that this technique consists in creating, in a phosphor screen, a latent electronic image, consisting of a metastable state of electronic excitation of the phosphor of the screen, generated by the .beta.-type ionizing radiation produced by the radioelements contained in the sample to be observed. The aforementioned latent image is then read, with destructive reading, by scanning the surface of the phosphor screen by a laser beam, which causes decay of the metastable state and has the effect of stimulating the emission of photons which are representative of the electronic image. The intensity of the photon emission is proportional to the energy locally supplied on the phosphor screen by the ionizing radiation.
The aforementioned technique makes it possible to detect the local energy supplied by the ionizing radiation and appears more satisfactory for producing the representation, that is to say the display, of the spatial distribution of radioactive elements generating energetic .beta. radiation than for radioactive elements generating less penetrating radiation. By way of example, the isotope .sup.32 P of phosphorus, which generates .beta. radiation with a mean energy of 600 keV, is approximately ten times easier to locate than the isotope .sup.35 S of sulfur, which emits .beta. rays whose mean energy is only of the order of 50 keV, by implementing the aforementioned technique. In contrast, when implementing this technique, the tritium isotope .sup.3 H is practically impossible to locate for two reasons. The mean energy of the .beta. radiation, whose value is on the order of 6 keV, is so low that either this radiation cannot pass through the protective layer normally used for the phosphor screen, or, in the absence of a protective layer, the very fragile and weakly sensitive screen can be used only once.